Conventionally, a metal can, a glass bottle and various plastic containers have been used as packaging containers. However, in view of light weight, impact resistance and cost, plastic containers prevail in a variety of applications.
However, even in the field of light packaging using a plastic material, preservability of the contents is an important factor. In order to kill microorganisms or spores, sterilization packaging is widely used. Examples of sterilization packaging include (1) a sterile filling method of heat sterilizing contents outside the container and filling the contents in a package subjected to disinfection treatment, (2) a heat sterilization method of filling contents into a container and then heat sterilizing the contents in a retort or boiling water, and (3) a hot filling method of filling heated contents into a package. Among these sterilization packaging techniques, in view of ease in operation and cost of the apparatus, the heat sterilization method is advantageous.
Popular light packages made of plastic, including flexible packages such as a pouch or a semi-rigid package such as a cup or a tray, comprise an intermediate layer of a metal foil having a gas barrier property and an inner surface layer of a heat sealable resin such as an olefin-base resin.
However, when colored contents are filled in the above-described plastic container and then subjected to heat sterilization, the plastic inner surface layer is often colored by the contents.
Coloring of the plastic inner surface layer by the contents not only involves loss of the dye or flavor component from the contents, but also affects quality and appearance and raises a problem with respect to quality retention. Furthermore, a container having a colored inner surface fails to assure users of the reliability of the package.